(1) Field of the Invention
The present invention relates to a permanent magnetic chuck capable of being switched over between a magnetically excited state and a non-excited state. More particularly, it is concerned with a switchable permanent magnetic chuck capable of being switched over between a magnetically excited state and a non-excited state by changing a relative positional relation between fixed permanent magnets and movable permanent magnets.
(2) Description of the Prior Art
As a conventional switchable permanent magnetic chuck there is known the permanent magnetic chuck disclosed in Japanese Patent Publication no. 16149/60. This conventional permanent magnetic chuck is provided with a housing having one open end; a face plate comprising a plurality of elongated magnetic pole members and a plurality of non-magnetic members, the pole members and the non-magnetic members being arranged in an alternately juxtaposed manner across the open end of the housing so as to cover this open end; a plurality of first permanent magnets fixed to the face plate; and a plurality of second permanent magnets fixed to a movable member which is enclosed in the housing so as to be movable in the transverse direction, namely, the juxtaposed direction, of the magnetic pole members and non-magnetic members.
In the above conventional permanent magnetic chuck, the first permanent magnets are each disposed between adjacent magnetic pole members of the face plate to give alternatively inverted magnetized directions in the transverse direction of the magnetic pole members so that the opposed faces between adjacent first permanent magnets are of the same magnetic pole. The movable permanent magnets are arranged in the moving direction of the movable member at predetermined intervals so that the opposed faces between adjacent movable permanent magnets are of the same magnetic pole, with a pole piece corresponding to the magnetic pole member being disposed therebetween.
According to the above conventional chuck, the face plate can be brought into a magnetically excited state by moving the second or movable permanent magnets which are magnetized in the same direction as the first or fixed permanent magnets, to positions just under the first permanent magnets. The face plate can be brought also into a non-excited state by moving the second permanent magnets which are magnetized in the direction opposite to the magnetized direction of the fixed permanent magnets, to positions just under the first permanent magnets.
In the magnetically excited state mentioned above, magnetic fluxes from the second permanent magnets are conducted to the magnetic pole members of the face plate not directly but through the pole pieces which are in contact with the magnetic pole faces of the movable permanent magnets. Because of such a construction, the foregoing conventional chuck has been disadvantageous in that the magnetic fluxes conducted from the second permanent magnets to the face plate in the magnetically exicted state leak in a relatively large amount, thus causing a large magnetic loss.